This invention relates to a noise reduction system for a compressor. More specifically, this invention relates to use of a branch type resonator to attenuate noise from a rotary type of compressor.
Compressors often generate undesirably high levels of noise. A particular rotary type of compressor often produces high frequency noise, especially noise in a narrow frequency band around 4 kHz. Unfortunately, human ears are generally most sensitive to frequencies around 4 kHz. The high pressure compressed gas inside the chamber is generally the primary noise source.
The compressor structure is vibrated primarily by the gas pressure inside the compression chamber. In a rotary compressor having a sliding vane and the type commonly used for refrigerators, the refrigerant gas pressure reaches up to 200 psi and maximum sound pressure level (SPL) is about 120 dB. Although the commonly used discharge mufflers attenuate the direct air-borne sound energy, the pressurized gas excites the mechanical structures (cylinder and shaft) and the high vibration frequency energy arrives on the casing to produce a high sound pressure level outside of the casing.
The most efficient way of attenuating the compressor noise is controlling the compressor gas spectrum directly. Any resonator type of device built in the discharge port works as a mechanical filter. Three types of resonators might be used for trying to reduce the noise: a muffler, an orifice type of branch, and a Helmholtz type of branch.
A muffler has a different cross-sectional area in order to expand gas abruptly. During the expansion process, a muffler reflects back the high frequency sound and transmits the low frequency sound energy. A muffler can be considered as a low-pass filter. Depending upon the requirements of maintaining compressor efficiency and the frequency characteristics of a particular compressor design, a muffler may require too large a structure for suitability.
An orifice acts as a high pass filter. The operating frequency range is quite broad and the device geometry is of the order of a wavelength corresponding to the frequency. As with the muffler, the orifice type of branch may be too large compared to the size of the compressor discharge port for a particular frequency noise signal.
As the muffler and orifice may be physically too big to be implemented in a particular type of rotary compressor, the Helmholtz resonator might be used. This resonator consists of a small size hole having a volume and a branch communicating with the main pipe or branch. If the incident wave is transmitted into the resonator, the resonator reflects back the certain frequency sound and transmits other than the reflected frequency sound. This resonator functions as a band-reject filter, the rejected band corresponding to the frequency of resonance of the Helmholtz resonator.
An example of the use of a Helmholtz resonator in a compressor is described in the proceedings of the 1984 International Compressor Engineering Conference at Purdue in a paper entitled "Analysis of Hermetic Rolling Piston Type Compressor Noise, and Countermeasures" by Sano et al., pages 242-250. In that article, the noise of a hermetic rolling position type compressor for room air conditioners was discussed. Additionally, the use of a Helmholtz resonator for a relatively broad band of noise attenuation was shown. In order to provide the broad frequency band noise reduction, the article shows a generally oval type of cavity for the Helmholtz resonator, although the actual geometry of the Helmholtz resonator is not discussed in detail.
Although the suppression of noise in rotary compressors has been viewed as a desirable feature, consideration of size constraints, increased cost from extra machining and/or assembly operations, and other factors have generally discouraged the use of certain types of noise reduction. Additionally, the use of particular types of noise reduction techniques has been ill-suited for compressors which have concentrated noise in a particular frequency band around 4 kHz. Further, some noise reduction techniques have been limited in application because their use tends to decrease the efficiency of the compressor. That is, such techniques provide only marginal noise improvement unless the techniques are carried sufficiently far as to result in significantly diminished compressor efficiency.